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Kinetics of ¹⁴C Distribution during Photosynthesis by Chloroplast Preparations Isolated from the Siphonous Alga Caulerpa simpliciuscula¹

Russell Grimwade School of Biochemistry, University of Melbourne, Parkville, Victoria 3052 Australia

The kinetics of ¹⁴C-labeling of compounds produced during photosynthesis by chloroplast preparations isolated from the green alga Caulerpa simpliciuscula were studied. After 10 minutes photosynthesis sucrose contained more ¹⁴C than any other product, and continued to accumulate radioactivity during the whole hour of incubation. Glucose-6-phosphate and alanine also behaved as end products and continued to accumulate label during the period. In these organelles, glucose-6-phosphate replaced triose phosphate as the main compound exported from the chloroplast during shorter periods of photosynthesis. When either glucose-6-phosphate or 3-phosphoglycerate was supplied to the isolated chloroplasts, they were metabolized, but were not converted to either sucrose or alanine. It is proposed that many of the differences in metabolism which distinguish these algal chloroplasts from those isolated from higher plants are due to their isolation in the form of cytoplasts, i.e. chloroplasts surrounded by a thin layer of extrachloroplastic material which is membrane-bound. The restriction of diffusion of intermediates from the chloroplast by this cytoplast membrane appears to be at least as important as the rather small amount of cytoplasm present in determining the properties observed.